High precision GPS kinematic positioning is widely used for many surveying and navigation applications on land, at sea and in the air. The distance from the mobile receiver to the nearest reference receiver may range from a few kilometers to hundreds of kilometers. As the receiver separation increases, the problem of accounting for distance-dependent biases grows and, as a consequence, reliable ambiguity resolution becomes an even greater challenge.
The standard mode of precise differential positioning uses one reference receiver located at a station whose coordinates are known, while determining a second receiver's coordinates relative to the reference receiver. In addition, the second receiver may be static or moving, and carrier phase measurements must be used to assure high positioning accuracy. This is the basis for pseudo-range-based differential global positioning system (GPS) (DGPS for short) techniques. However, for high precision applications, the use of carrier phase data comes at a cost in terms of overall system complexity because the measurements are ambiguous, requiring that ambiguity resolution (AR) algorithms be incorporated as an integral part of the data processing software.
Such high accuracy techniques result from progressive research and development (R&D) innovations, subsequently implemented by GPS manufacturers in top-of-the-line “GPS surveying” products. Over the last decade, several significant developments have resulted in the high accuracy performance also being available in “real-time”—that is, in the field, immediately following measurement, and after the data from the reference receiver has been received by the (second) field receiver for processing via a data communication link (e.g., very high frequency (VHF) or ultra high frequency (UHF) radio, cellular telephone, frequency modulation (FM) radio sub-carrier or satellite communication link). Real-time precise positioning is even possible when the GPS receiver is in motion through the use of “on-the-fly” (OTF) AR algorithms. These systems are commonly referred to as “real-time kinematic” (RTK) systems, and make feasible the use of GPS-RTK for many time-critical applications, e.g., machine control, GPS-guided earthworks/excavations, automated haul truck operations, and other autonomous robotic navigation applications.
If the GPS signals were continuously tracked (loss-of-lock never occurred), the integer ambiguities resolved at the beginning of a survey would be valid for the whole GPS kinematic positioning span. However, the GPS satellite signals are occasionally shaded (e.g. due to buildings in “urban canyon” environments), or momentarily blocked (e.g. when the receiver passes under a bridge or through a tunnel), and in these circumstances the integer ambiguity values are “lost” and must be re-determined or re-initialized. This process can take from a few tens of seconds up to several minutes with present OTF AR techniques. During this “re-initialization” period, the GPS carrier-range data cannot be obtained and there is “dead” time until sufficient data has been collected to resolve the ambiguities. If GPS signal interruptions occur repeatedly, ambiguity re-initialization is, at the very least, an irritation, and, at worst, a significant weakness of commercial GPS-RTK positioning systems. In addition, the longer the period of tracking required to ensure reliable OTF AR, the greater the risk that cycle slips occur during the crucial (re-)initialization period. A loss of lock of a receiver phase lock loop causing a sudden integer number of cycles jump in a carrier phase observable is known as a cycle slip. Receiver tracking problems or an interrupted ability of the antenna to receive satellite signals causes the loss of lock. As the receiver separation increases, the problems of accounting for distance-dependent biases grow and, as a consequence, reliable ambiguity resolution (or re-initialization) becomes an even greater challenge due to:
Residual biases or errors;
Observation span determination; and
Kinematic positioning results quality control.
Residual biases or errors after double-differencing can only be neglected for AR purposes when the distance between two receivers is less than about 10 km. For distances greater than 10 km, the distance-dependent biases, such as orbit bias, ionospheric delay and tropospheric delay, become significant problems.
Determining how long an observation span should be for reliable AR is a challenge for real-time GPS kinematic positioning. The longer the observation span required, the longer the “dead” time during which precise position determination is not possible. This can happen at the ambiguity initialization step if a GPS survey is just starting, or at the ambiguity re-initialization step if the GPS signals are blocked, such as due to cycle slips or data interruptions.
Quality control of GPS kinematic positioning results is a critical issue and is necessary during all processes: data collection, data processing and data transmission. Quality control procedures are not only applied for carrier phase-based GPS kinematic positioning, but also for pseudo-range-based DGPS positioning. However, quality control or validation criteria for AR, for precise GPS kinematic positioning, is a significant challenge.
There is a need in the art for an improved method of processing GPS signals to improve decorrelation and parameter tuning capabilities for high accuracy, reliable position determination.